Method for transferring aqueous polymer dispersions from one container to another container

ABSTRACT

A method of transferring an aqueous polymer dispersion with a temperature ≧50° C. from a vessel 1 via a connecting line to a vessel 2 is described.

The present invention relates to an improved method of transferring anaqueous polymer dispersion with a temperature ≧50° C. from a vessel 1via a connecting line to a vessel 2, wherein the vessel 2, before andduring the transfer, contains water vapor with a partial pressure of≧100 mbar.

Aqueous polymer dispersions are common knowledge. They are fluid systemswhich constitute as their disperse phase polymer coils consisting of aplurality of interlooped polymer chains, known as the polymer matrix orpolymer particles, in disperse distribution in the aqueous dispersionmedium. The diameter of the polymer particles is frequently in the rangefrom 10 to 5000 nm. Aqueous polymer dispersions are used in a largenumber of industrial applications in the form of binders, in paints orrenders, for example, in coatings for leather, paper or polymer films,and as components in adhesives.

Aqueous polymer dispersions are obtainable in particular byfree-radically initiated aqueous emulsion polymerization ofethylenically unsaturated monomers. This method has been the subject ofmany prior descriptions and is therefore adequately known to the skilledworker [cf., e.g., Encyclopedia of Polymer Science and Engineering, Vol.8, pages 659 to 677, John Wiley & Sons, Inc., 1987; D.C. Blackley,Emulsion Polymerisation, pages 155to 465, Applied Science Publishers,Ltd., Essex, 1975; D.C. Blackley, Polymer Latices, 2nd Edition, Vol. 1,pages 33 to 415, Chapman & Hall, 1997; H. Warson, The Applications ofSynthetic Resin Emulsions, pages 49 to 244, Ernest Benn, Ltd., London,1972; D. Diederich, Chemie in unserer Zeit 1990, 24, pages 135 to 142,Verlag Chemie, Weinheim; J. Piirma, Emulsion Polymerisation, pages 1 to287, Academic Press, 1982; F. Hölscher, Dispersionen synthetischerHochpolymerer, pages 1 to 160, Springer-Verlag, Berlin, 1969 and thepatent DE-A 40 03 422]. Free-radically initiated aqueous emulsionpolymerization normally takes place in such a way that the monomers,often with the use of dispersing aids, are dispersed in the aqueousmedium and polymerized by means of at least one free-radicalpolymerization initiator.

Suitable reaction temperatures for the free-radical aqueous emulsionpolymerization cover the entire range up to 170° C.; temperatures offrom 70 to 120° C., preferably from 80 to 100° C., and with particularpreference >85 to 100° C., however, are preferably employed.Free-radical aqueous emulsion polymerization may be conducted at apressure of less than, equal to or greater than 1 bar (absolute), sothat the polymerization temperature may exceed 100° C. and may be up to170° C. Volatile monomers such as ethylene, butadiene or vinyl chlorideare preferably polymerized under elevated pressure. In this case thepressure may be 1.2, 1.5, 2, 5, 10, 15 bar or even higher. Whereemulsion polymerizations are conducted under subatmospheric pressure,the pressures set are 950 mbar, frequently 900 mbar, and often 850 mbar(absolute). Free-radical aqueous emulsion polymerization isadvantageously conducted at 1 bar (absolute) under an inert gasatmosphere, such as under nitrogen or argon, for example.

In many cases, in the aqueous polymer dispersions obtained, the residualamounts of unreacted monomers are lowered by chemical and/or physicalmethods which are likewise known to the skilled worker [see, forexample, EP-A 771328, DE-A 19624299, DE-A 19621027, DE-A 19741184, DE-A19741187, DE-A 19805122, DE-A 19828183, DE-A 19839199, DE-A 19840586and19847115], the polymer solids content is adjusted to a desired level bydilution or concentration, or further customary additives, such asbactericides or foam suppressant additives, for example, are added tothe aqueous polymer dispersions.

On the industrial scale in particular it is advantageous for the processsteps following the aqueous emulsion polymerization not to be carriedout in the polymerization reactor, which is extensively equipped interms of its apparatus and the measurement and control technology, butinstead in a more simply equipped reaction vessel, such as a so-calledblow down reactor or formulating vessel. For this purpose, after the endof the polymerization reaction, in which the ethylenically unsaturatedmonomers have been reacted to an extent ≧90% by weight, preferably ≧95%by weight, and in particular ≧98% by weight, the resulting aqueouspolymer dispersion is transferred without cooling, via a connectingline, into the downstream reaction vessel. In this context it has beenproposed to carry out the transfer by pumping the aqueous polymerdispersion into the reaction vessel or forcing it into the reactionvessel by applying a nitrogen or water vapor overpressure in thepolymerization reactor. In particular it is advantageous for thisreaction vessel to be sited at a lower level than, or even beneath, thepolymerization reactor. In this case the aqueous polymer dispersion maybe drained simply into the reaction vessel, utilizing gravity. So thatthe transfer of the aqueous polymer dispersion to the reaction vessel isparticularly rapid and the polymerization reactor is thus made availableas quickly as possible again for the preparation of the next polymerdispersion, the aqueous polymer dispersion is often both pumped andforced into a lower-lying reaction vessel.

In order that there is no buildup in this second reaction vessel of anoverpressure which would again slow down or delay the transferoperation, this vessel is either vented to the atmosphere (for example,by way of an offgas scrubber or muffle flare) or evacuated to a pressure<100 mbar (absolute) prior to the transfer. However, this process hasbeen found disadvantageous in that, during the transfer, andirrespective of whether the aqueous polymer dispersion is drained off,pumped and/or forced in circulation, unwanted polymer coagulum is formedin the aqueous polymer dispersion. This polymer coagulum, with aparticle size ranging from a few micrometers up to a few centimeters,may reduce the clarity of the films formed from the polymers (gelspecks), reduce the binding power of the polymers in paint, render,coating, and adhesive formulations, or lead to disruptive deposits inproduction plant and processing machines.

It is an object of the present invention to provide a method oftransferring an aqueous polymer dispersion with a temperature ≧50° C.from a vessel 1 via a connecting line to a vessel 2 while preventing orat least reducing the formation of polymer coagulum.

We have found that this object is achieved by the method describedabove.

For the purposes of this document, vessels are reactors, stirred tanks,interim containers and storage containers, and also freight containers,drums, cannisters and cans, etc., of any of a very wide variety of sizesand shapes. The material from which the vessels are constructed is notimportant. For example, a very wide variety of alloyed and unalloyedgrade steels, chemically resistant stainless steel types, such as 1.4541and 1.4571 steel, and also aluminum, with or without a very wide varietyof internal coatings, such as enamel, silver, zinc and tin or plastics,such as PTFE and coatings, or plastics, such as polyethylene,polypropylene, polystyrene, polyacrylamide, and glass fiber reinforcedsynthetic resins, for example, are employed. These vessels may containany of a very wide variety of internals, such as stirrers, heatexchangers, heating coils and/or cooling coils, flow disruptors andsensors, for example, and also a wide variety of connections andopenings, which may be closeable by closures, such as ballcocks, valvesof myriad construction, and also screw lids, etc.

In accordance with the invention it is advantageous if the temperatureof the inner surfaces of the connecting line and of the vessel 2 withwhich the aqueous polymer dispersion comes into contact during thetransfer is less than or equal to the temperature of the aqueous polymerdispersion in vessel 1.

The temperature of the aqueous polymer dispersion is often ≧50° C., ≧60°C., ≧70° C., ≧80° C. or ≧90° C. and also ≦170° C., ≦150° C., ≦130° C.,≦110° C., ≦100° C., ≦95° C. or ≦90° C., and all values inbetween. Ingeneral, however, the temperature is situated between 50 and 100° C.,frequently between 60 and 95° C., and in particular between 65 and 90°C.

Essential to the method is that the water vapor partial pressure invessel 2 before and during the transfer of the aqueous polymerdispersion is ≧100 mbar. It is of advantage if the water vapor partialpressure in vessel 2 is ≧70%, ≧80% or ≧90% and ≦100% of the water vaporpartial pressure of the aqueous polymer dispersion in vessel 1.

The water vapor partial pressure of the aqueous polymer dispersion invessel 1 is in principle dependent on the water content, temperature,and other constituents of the aqueous polymer dispersion. It may bedetermined simply, in a first approximation, by introducing, say, theaqueous polymer dispersion at 20° C. into a closeable container to whicha pressure measuring device is connected, and evacuating the containerto a self-determined final pressure, 30 mbar (absolute) for example. Theaqueous polymer dispersion is subsequently heated to the temperature ithas in vessel 1. The equilibrium pressure which becomes established inthe container, minus the residual pressure obtaining beforehand, thengives the water vapor partial pressure of the aqueous polymer dispersionat the specified temperature. It is of importance that the water vaporpartial pressure of an aqueous polymer dispersion at a given temperaturecorresponds in good approximation to the water vapor partial pressure ofpure water at the same temperature. The temperature dependency of thewater vapor partial pressure of water is known to the skilled worker orcan be looked up by said skilled worker in familiar reference works, anexample being the Handbook of Chemistry and Physics, 80^(th) Edition,1999–2000, Sections 6–10 to 6–11, CRC-Press.

The water vapor partial pressure of vessel 2 may be set, for example, byheating the tank wall and the internals of the empty vessel 2 to atemperature less than or equal to the temperature of the aqueous polymerdispersion in vessel 1 and introducing water vapor with the same or alower temperature to displace the gas present in the vessel 2,frequently air or nitrogen. Before the transfer of the aqueous polymerdispersion, any water vapor condensate produced can be separated off.

It is particularly advantageous if before the transfer the empty vessel2 is evacuated to a pressure of ≦100 mbar, ≦90 mbar or ≦80 mbar(absolute) and then by introduction of water vapor a water vaporpressure of ≧100 mbar, ≧200 mbar, ≧300 mbar, ≧400 mbar, ≧500 mbar, ≧600mbar or ≧700 mbar and ≦1000 mbar, ≦900 mbar, ≦800 mbar or ≦700 mbar andall values inbetween is set. Frequently, in vessel 2 before the transfera water vapor partial pressure of from 300 to 700 mbar is set. It isimportant that the transfer of the aqueous polymer dispersion normallytakes place without pressure compensation or deairing of the vessel 2.

In another embodiment of the method of the invention, first theconnecting line is filled with the aqueous polymer dispersion and thenthe aqueous polymer dispersion is transferred to the vessel 2. For thispurpose it is necessary for there to be a blocking means, such as a ballcock, valve or sliding blocker, present at least at both ends of theconnecting line, i.e., in the vicinity of the discharge opening of thevessel 1 and/or of the inlet opening of the vessel 2. In the case ofrelatively long connecting lines, furthermore, a venting line isadvantageous. In accordance with the invention, then, first theconnecting line is filled with aqueous polymer dispersion, before thetransfer, by opening of the blocking means on the discharge opening ofthe vessel 1 and, where present, of the venting line. Only thereafter isthe blocking means opened at the inlet opening of the vessel 2 and theaqueous polymer dispersion transferred into the vessel 2. Thisembodiment is especially effective when the vessel 2 has anunderpressure.

The method of the invention prevents or reduces advantageously theformation of disruptive polymer coagulum in aqueous polymer dispersionsduring their transfer from one vessel to another. Likewise prevented orreduced are polymer deposits in the connecting line and on the inletopening of the vessel 2. The present process is easy to realizeindustrially and ensures optimum utilization of polymerizationcapacities in the preparation of aqueous polymer dispersions.Furthermore, the process described is in principle not restricted toaqueous polymer dispersions, but instead may be employed generally withaqueous or nonaqueous polymer dispersions, solutions or suspensions.Where in this case, for example, a nonaqueous medium is used to take upthe polymers, however, it is the vapor of the nonaqueous medium used totake up the polymers, rather than water vapor, which is to be used.

EXAMPLES Analysis

The number-average particle diameter of the polymer particles wasdetermined by dynamic light scattering on an aqueous dispersion with aconcentration of from 0.005 to 0.01 percent by weight at 23° C. using anAutosizer IIC from Malvern Instruments, Great Britain. The reportedfigure is the average diameter of the cumulant evaluation (cumulantz-average) of the measured autocorrelation function (ISO Standard 13321).

The solids content was determined by drying an aliquot of the aqueouspolymer dispersion in a drying oven at 140° C. for 6 hours. Two separatemeasurements were conducted. The reported value represents the averageof the two measurements.

The amount of coagulum was determined by filtration through a metalsieve with a mesh size of 45 μm. For this, 100 g of the aqueous polymerdispersion were filtered through the 45 μm sieve, which was weighedbefore filtration, at from 20 to 25° C. (room temperature). Followingfiltration, the sieve was rinsed with a little deionized water and thendried to constant weight in a drying oven at 100° C. under atmosphericpressure. After cooling to room temperature, the sieve was weighedagain. The amount of coagulum was found from the difference between theindividual weighings, based in each case on the amount of aqueouspolymer dispersion filtered.

Transfer tests

The transfer tests were carried out with an acrylate dispersion whichhad a solids content of 50.3% by weight and whose polymer particles wereobtained by polymerizing a monomer mixture containing 87.2% by weight2-ethylhexylacrylate, 10.8% by weight acrylonitrile and 2.0% by weightacrylic acid. The number-average particle diameter was 370 nm. Theamount of coagulum >45 μm was found to be 0.06% by weight.

The apparatus used for the transfer tests consisted of two heatable 2 1double-walled stainless steel vessels, vessel 1 being equipped with ananchor stirrer, thermometer, filling port and one connection each to avacuum system and to a water vapor system, and vessel 2 being equippedwith one connection each to a vacuum system, a water vapor system and anair supply system, an anchor stirrer, a thermometer, and a dischargeport. Between the vessels there was a stainless steel transfer lineapproximately 70 cm in length which had an internal diameter of 6 mm anda blocking means at both ends. The blocking means close to the outletopening of vessel 1 is referred to as “outlet tap” below and theblocking means close to the inlet opening of vessel 2 is referred to as“inlet tap”. Vessel 1 was arranged above vessel 2 with a lateral offset.The outlet opening of vessel 1, which opened into the transfer line, waslocated in the base of said vessel 1, while the inlet opening of vessel2, which opened into the transfer line, was situated in the lid of saidvessel 2. Prior to the transfer tests, both vessels and the transferline were empty and dry.

Example 1

1.5 1 of the abovementioned aqueous polymer dispersion were introducedinto vessel 1 at 85° C. with stirring (100 revolutions per minute).Vessel 2 and the transfer line (inlet tap on vessel 2 open and outlettap on vessel 1 closed) were evacuated to 50 mbar (absolute) and theinner vessel walls were heated to 85° C. Subsequently, the vacuum lineand the inlet tap were closed and water vapor was introduced into vessel2 up to an internal vessel pressure of 500 mbar (absolute). Thereafterthe outlet tap was slowly opened, so that the transfer line was slowlyflooded with aqueous polymer dispersion. Subsequently 2 bar of watervapor were injected into vessel 1 and the inlet tap was opened in such away that transfer of the aqueous polymer dispersion into vessel 2 took30 seconds. Immediately after the emptying of vessel 1, the water vaporline and the outlet tap, and only then the inlet tap, were closed. Thestirrer in vessel 2 was subsequently switched on (100 revolutions perminute) and the aqueous polymer dispersion was stirred for 60 seconds.Thereafter the air supply line was opened and the aqueous polymerdispersion was cooled to room temperature.

The amount of coagulum >45 μm in the aqueous polymer dispersion obtainedafter transfer was found to be 0.31% by weight.

Example 2

Example 2 was carried out as for example 1 except that water vapor wasintroduced into vessel 2 up to an internal vessel pressure of 550 mbar(absolute).

The amount of coagulum >45 μm in the aqueous polymer dispersion obtainedafter transfer was found to be 0.22% by weight.

Example 3

Example 3 was carried out as for example 1 except that water vapor wasintroduced into vessel 2 up to an internal vessel pressure of 600 mbar(absolute).

The amount of coagulum >45 μm in the aqueous polymer dispersion obtainedafter transfer was found to be 0.17% by weight.

Example 4

Example 4 was carried out as for example 1 except that water vapor wasintroduced into vessel 2 up to an internal vessel pressure of 630 mbar(absolute).

The amount of coagulum >45 μm in the aqueous polymer dispersion obtainedafter transfer was found to be 0.06% by weight.

Comparative Example

The comparative example was carried out as for example 1 except that nowater vapor was introduced into vessel 2.

The amount of coagulum >45 μm in the aqueous polymer dispersion obtainedafter transfer was found to be 0.66% by weight.

1. A method of transferring an aqueous polymer dispersion, whichcomprises: transferring the aqueous polymer dispersion with atemperature ≧50° C. from a vessel 1 via a connecting line to a vessel 2,wherein the vessel 2, before and during the transfer, contains watervapor with a water vapor partial pressure ≧100 mbar.
 2. A method asclaimed in claim 1, wherein the temperature of the inner surfaces of theconnecting line and of the vessel 2 with which the aqueous polymerdispersion comes into contact during the transfer is less than or equalto the temperature of the aqueous polymer dispersion in vessel
 1. 3. Amethod as claimed in claim 1, wherein the water vapor partial pressurein vessel 2 is greater than or equal to 70% of the water vapor partialpressure of the aqueous polymer dispersion in vessel
 1. 4. A method asclaimed in claim 1, wherein before the transfer occurs the vessel 2 isevacuated to a pressure of ≧100 mbar (absolute), then in vessel 2 awater vapor partial pressure of from 300 to 700 mbar is set andthereafter the aqueous polymer dispersion is transferred to the vessel 2without pressure compensation.
 5. A method as claimed in claim 1,wherein first the connecting line is filled with the aqueous polymerdispersion and then the aqueous polymer dispersion is transferred to thevessel
 2. 6. A method of transferring a polymer dispersion, solution orsuspension, which comprises: transferring a polymer dispersion, solutionor suspension from a vessel 1 via a connecting line to a vessel 2,wherein the vessel 2, before and during the transfer, contains vapor ofthe liquid medium which was used to take up the polymer, and the vaporpressure of the liquid medium is ≧70% of the equilibrium vapor pressureof the liquid medium at the temperature possessed by the polymerdispersion, solution or suspension during the transfer.
 7. A method asclaimed in claim 2, wherein the water vapor partial pressure in vessel 2is greater than or equal to 70% of the water vapor partial pressure ofthe aqueous polymer dispersion in vessel
 1. 8. A method as claimed inclaim 2, wherein before the transfer occurs the vessel 2 is evacuated toa pressure of ≧100 mbar (absolute), then in vessel 2 a water vaporpartial pressure of from 300 to 700 mbar is set and thereafter theaqueous polymer dispersion is transferred to the vessel 2 withoutpressure compensation.
 9. A method as claimed in claim 3, wherein beforethe transfer occurs the vessel 2 is evacuated to a pressure of ≧100 mbar(absolute), then in vessel 2 a water vapor partial pressure of from 300to 700 mbar is set and thereafter the aqueous polymer dispersion istransferred to the vessel 2 without pressure compensation.
 10. A methodas claimed in claim 2, wherein first the connecting line is filled withthe aqueous polymer dispersion and then the aqueous polymer dispersionis transferred to the vessel
 2. 11. A method as claimed in claim 3,wherein first the connecting line is filled with the aqueous polymerdispersion and then the aqueous polymer dispersion is transferred to thevessel
 2. 12. A method as claimed in claim 4, wherein first theconnecting line is filled with the aqueous polymer dispersion and thenthe aqueous polymer dispersion is transferred to the vessel
 2. 13. Amethod as claimed in claim 1, wherein vessel 2 is empty before thetransfer of the aqueous polymer dispersion.
 14. A method as claimed inclaim 1, which is discontinuous.
 15. A method as claimed in claim 6,wherein vessel 2 is empty before the transfer of the aqueous polymerdispersion.
 16. A method as claimed in claim 6, which is discontinuous.